Starch is a naturally occurring polymer made up of simple sugars and is obtained by processing plant materials. The plant materials from which starch is derived include, but are not limited to, corn, wheat, potato and tapioca. Of these plant materials, corn is one of the most widely used sources for starch in North America.
Starch is used in a wide number of applications, which include both industrial and private settings. These uses include food products, papermaking, corrugated boxes, glue, baby powder and textiles. In some of these applications, a dry starch is used, while in others, a cooked starch is employed.
Starch has a number of unique properties that make it suitable for a variety of applications. One of starch's many properties is its ability to form a viscous fluid upon application of energy and water. The viscous fluid can be applied to substrates such as paper and textiles, whereby it increases the stiffness of the material.
While starch has a number of unique properties, it has some properties that can limit its usefulness in certain applications. For example, cooked starch has a tendency to congeal and fall out of solution if the paste hold temperature drops below a certain level. This tendency to drop out of solution is called "retrogradation." Once a starch retrogrades, it is difficult to get the cooked starch back into solution.
Another property that limits the use of cooked starch in certain applications is the tendency to form amylose crystals under certain pH and temperature conditions. Amylose is a component of starch that is essentially a straight chain of simple sugars. The other major type of starch is amylopectin, which is a branched starch molecule. Amylose crystals are undesirable because, among other reasons, of their limited ability to bind to substrates. A cooked starch paste has a greater tendency to form amylose crystals under low pH conditions (7.5 pH or less) and at temperatures between 153 and 193.degree. F. (Wurzburg, O.B.,Modified Starches: Properties and Uses, CRC Press, Inc., Boca Raton, Florida, 1986, p. 226).
To expand the use of cooked starch applications, manufacturers have chemically altered, or modified, the uncooked starch molecule. In the case of retrogradation or amylose formation, the uncooked starch has been modified by attaching "bulky" side chemical groups to the starch molecule. The bulky side groups keep adjacent starch molecules from aligning in close proximity and thus hinder the tendency to coalesce after cooking. Similarly, the bulky side groups also reduce the tendency to form amylose crystals.
Typically, the starch modification is accomplished by treating an aqueous uncooked starch granule suspension with a reagent. Depending on the product being formed, the modification can be accomplished by treating with reagents such as, but not limited to, ethylene oxide, hydrochloric acid, hydrogen peroxide and sodium hypochlorite.
One common technique used by industry to add carboxyl groups onto a starch molecule (or chain) is by reacting starch with an oxidant. These oxidants include, but are not limited to, sodium hypochlorite and calcium hypochlorite. It is noted that the oxidation reaction causes additional changes in the starch molecule such as causing the cooked starch paste to become less viscous because it "clips" the length of the chain. When either sodium or calcium hypochlorite is used as an oxidant, the reaction is run under alkaline conditions, which favors the production of the bulkier carboxylic acid side groups over ketone side groups (Wurzburg, pps. 23-29). The alkaline conditions are obtained by adding sodium hydroxide solution. At the end of the reaction, the excess alkalinity is neutralized with acid.
Other methods of oxidizing starches include reacting starch with: ozone in an aqueous medium (PCT Patent Application WO935890); a reagent producing oxoammonium ion, in particular 2,2,6,6-tetramethylpiperidin-1-oxyl, in the presence of an oxidizing agent or oxidizing enzyme such as laccase (PCT Patent Application WO9923240); and halogen or hypohalite in the presence of di-tertiary alkyl nitroxyl and an alkali metal or alkaline earth metal bromide as catalysts (German Patent No. 19746805). Chemically-mechanically (C-M) gelatinized starch may also be derivatized by reaction with hypochlorite at an alkaline pH. C-M gelatinized starch is starch that has been heated and subject to a shearing force to produce an essentially dispersed and homogeneous starch paste (U.S. Pa. No. 4,579,944). Other procedures employ a gelatinization inhibitor such as sodium chloride or sodium sulfate, and react the starch with the derivatizing agent at an alkaline pH for a period of time up to 20 hours.
As mentioned earlier when discussing properties that limit the use of cooked starch in certain applications, amylose crystals have a greater tendency to form at temperatures between 153 and 193.degree. F. and at pH's less than 7.5. Based on these phenomena, others have focused their work on preventing amylose formation in cooked starch through the means of controlling temperature and/or pH.
The most straightforward way to ensure that the pH in the cooked starch is in the appropriate range is to control it using standard industrial techniques. For example, the pH control would likely be a pH probe (or multiple pH probes) in the cooked starch tanks which control the addition of either an acid or a caustic substance. In this case, if the pH was too low, an alkaline sodium hydroxide solution could be added to raise the pH.
While the pH control method has some advantages, it has a number of disadvantages. One disadvantage is the control of the pH is dependent on the reading of the pH sensors in the cooked starch tanks. If the reading from the sensor is in error, the resultant addition of acid or caustic will be in error. For example, it is well known that the readings from a pH probe will drift with time. This drift is caused by a number of things, including the physical deterioration with time of the probe, build-up of material on the probe, short circuiting of the wires leading from the pH sensor to the controller, etc. Because of the critical nature of the pH sensors in relation to preventing the formation of amylose crystals, the probes need a relatively high degree of maintenance to ensure they are working properly. This high degree of maintenance adds to the cost of producing the material, thereby raising the cost of the product.
Another way to reduce the tendency to form amylose crystals is by heating and maintaining the temperature of the cooked starch above 193.degree. F. (89.degree. C.). Normally, cooked starch is kept at an elevated temperature until it is used. However, the temperature required for prevention of amylose crystals is higher than typically required for the cooked starch paste. Additionally, the higher temperature will increase the rate of evaporation for the water, which will increase the solids content of the cooked starch slurry. Thus, using this technique to reduce amylose formation will increase the energy costs by having to heat the slurry to a higher temperature and has the possibility of changing the cooked starch solids concentration.
Alternatively, to reduce the tendency to form amylose crystals, there exists the option of keeping the temperature of the starch paste at less than 153.degree. F. However, in certain applications this is not a preferred option because the lower temperature paste does not penetrate into the paper which can cause a lower strength in the plane of the paper.
It is apparent from the above that there exists a need in the art for a method which produces a cooked starch which has a reduced tendency to form amylose crystals but at the same time do it in an efficient cost effective manner. It is the purpose of this invention to fulfill this need and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.